Discriminating land mines from natural backgrounds by depolarization

Results from an experimental polarimetric investigation of 7 different types of land mines and 3 types of plants with the aim to explore the possibility in discriminating surface land mines from natural backgrounds are presented. The reflection Mueller matrices at both specular and non-specular angles at 405 nm and 1570 nm laser illumination were collected. Also included in this study are reflection spectra from the mines taken from 400 to 2500 nm, as well as actual images of surface land mines hidden in a natural environment during different weather conditions. The mines had a reflection coefficient between 5-15 % with peak values around 510 nm due to the embedded green pigment. The mines were found to be less reflective in wet compared to dry conditions. The polarimetric study revealed that the samples had similar retardance and diattenuation values for small incident angles, but that discrimination between the samples could be made by monitoring the depolarization of the incident light for several incident angles, as a function of the angular distribution of scattered light. The land mines generally experience less depolarization than the investigated plants, specifically for specular angles around 1570 nm where the mines act as a non-depolarizing sample with depolarization index close to 1.0. The depolarization index is significantly smaller for specular angles from the plants, becoming 0.4 or below. Both plants and mines experience more depolarization for non-specular angles. A non-specular angular scan with a constant bi-static angle resulted in a Lorentzian shaped depolarization index curve, with characteristic differences in the fitted line-shape parameters. Remote laser based polarimetry might thus be a promising supplementary technique in recognizing surface mines or other related man-made objects from a natural background. Conclusively, the depolarization index as a function of angular distribution of scattered light along with its wavelength dependence is a metric that produce significant differences in the polarimetric signatures.